Piezelectric measuring technology explained

Piezoelectric sensors now number among the key technologies that determine business success for industrial manufacturers. The experts at Kistler explain how this technology functions and the benefits.

Measurement technology based on the piezoelectric principle delivers a remar-kable increase in process reliability within a company’s production chain, coupled with a sustained improvement in productivity – opening up the way to zero defect production in joining, assembly and testing.

Piezoelectric measurement tech-nology is especially suitable as a means of optimising and controlling production processes by measuring force, pressure, acceleration and torque. When a mechanical load is applied to the quartz crystal used in the sensor technology, the crystal generates a charge signal that is directly proportional to the force acting on it.

The benefit of this is that, due to the high rigidity of the crystal, the measuring deflection is low – usually within the range of a few micrometers. When the processes under test are fast and dynamic, the high natural frequency of the quartz proves to be an advantage. Various piezo-effects are differentiated according to the position of the polar crystal axes in relation to the acting force:

Longitudinal effect: A charge is developed on the surfaces to which the force is applied, where it can be measured via electrodes. Piezoelectric elements with the longitudinal effect are sensitive to compression forces, making them especially suitable for simple and sturdy sensors that measure forces.

Shear effect: The piezoelectric sensitivity involved in the shear effect is independent of the size and shape of the piezoelectric element. Shear-sensitive piezo elements are used for sensors that measure shear forces, torque and strain, and also for accelerometers. They are suitable for use in sensors that must perform well even in temperature changes.

Transverse effect: Exploiting the transverse effect makes it possible to obtain a greater charge yield through appropriate shaping and alignment of the piezoelectric elements. Elements can be used for high-sensitivity pressure, strain and force sensors.

Charge amplifiers convert the charge produced by a piezoelectric sensor into a proportional voltage. The amplifier acts as an integrator, constantly compen-sating the electrical charge produced by the sensor on the range capacitor, in proportion to the acting meas-urand.

Quartz sensors allow both direct and indirect force measurements. For direct measurements, the sensor is positioned fully in the force flux, and it measures the entire force. This approach yields high measurement accuracy that is virtually independent of the force application point. If the sensor cannot be positioned directly in the force flux, it will only measure part of the force; the remainder passes through the structure in which it is mounted.

Indirect force measurement

With indirect force measurement, strain sensors are used to measure the process force indirectly via the structural strain. Quartz sensors are exceptionally stable, rugged and compact. These attributes account for their widespread use in R&D as well as production and industrial testing technology.

Measurements with piezoelectric quartz force sensors deliver a whole series of benefits for dynamic and quasi-static measurements. They can be used for multiple measuring ranges, because the force is measured directly via the sensor elements rather than indirectly through the deformation of a structure. This feature explains why these measuring elements can measure across several decades, with no need to exchange them when measuring different forces.

Piezo sensors offer another benefit as regards overload protection: piezoelectric quartz force sensors react to load, not strain. This means that virtually no displacement occurs during the measurement. Massive overloading is possible with no risk of pressure-induced destruction. Even if the sensor is overloaded beyond its permitted measuring range, no damage will occur, nor will there be any zero point offsets, fatigue or linearity changes.

Piezoelectric sensors also offer added value in terms of stable sensitivity: quartz force sensors feature a solid-state design, and quartz does not exhibit any signs of aging. Quartz elements are not displaced under load, so sensitivity changes are unlikely – eliminating the need for frequent calibrations. These advantages all save time and money.

Dimensions are often a critical factor in the choice of a force sensor. In this regard too, piezoelectric quartz force sensors have the advantage: they require minimal space, so the mass load they add in dynamic investigations is negligible

Yet more advantages of piezoelectric quartz technology include high output voltages and wide temperature ranges, as well as low acquisition and life-cycle costs.

So that the data captured by highly sensitive piezo sensors can actually be used, it is visualised, evaluated and documented in suitable monitoring systems. Systems of this sort have to be integrated into production so that the quality of the manufactured products can be verified and/or assessed. Where many individual components are assembled to form one single product, each component must already have been tested by the suppliers. In assembly technology, Kistler's maXYmos monitoring system reliably tracks the entire production process, optimising the process towards the goal of zero defect production.